The present invention relates to syringes, and more particularly to syringes which are useful for certain medical procedures.
In recent years, gas analysis of arterial blood has become one of the most important laboratory tests in the management of patients with respiratory and metabolic disorders. However, the collection of a satisfactory arterial blood sample from a patient for analysis has posed a number of difficulties. Initially, in some patients it may be somewhat difficult to ascertain whether the collection device has received arterial or venous blood without measuring for the relatively high arterial pressures during collection. Second, the collection device should minimize contact of the blood sample with air since the air may affect the results of gas analysis. It is also desirable that the sample should not be collected in the presence of a vacuum, since it is believed that the vacuum may modify the gas characteristics of the sample. Finally, the device must prevent coagulation of the blood sample, and should be in a suitable form to permit closure of the sample to air and chilling during the period of time between collection and analysis.
In the past, plastic and ground glass syringes with a needle have been commonly used to collect the samples. However, prior syringes with rubber plugs of conventional design have proven deficient for such purposes due to the relatively high resistance between the syringe plunger and barrel. The plunger resistance in conventional plastic syringes prevents movement of the plunger responsive to arterial pressure alone, and requires that the plunger be manually withdrawn, thus creating an undesirable vacuum in the syringe chamber during collection. Further, since the plungers of such plastic syringes are not sufficiently mobile to move under arterial pressure, they do not provide an indication whether arterial or venous blood is being collected. Although the plungers of the ground glass syringes may be used to detect arterial pressure, the ground glass syringes are excessively expensive relative the plastic syringes.
During other medical procedures, such as an epidural anesthesia procedure, it is necessary to position the tip of a needle at a relatively precise position inside the patient's body. During this particular procedure, the needle tip should be located in the potential epidural space where the body pressure during insertion of the needle is normally slightly negative. If the needle tip has been advanced too far into the body, it projects through the dura mater into the subarachnoid space where the body pressure is positive. A ground glass syringe may be utilized in this procedure to determine the location of the needle tip through use of the relative body pressures, but conventional plastic syringes have not been sufficiently responsive to pressures for this purpose. Since syringes are normally considered a disposable item, it is desirable to reduce their cost to the hospital and patient below that of the relatively expensive ground glass syringes.